SELECTIVE INDUCTION OF AN EMBRYONIC FIBRONECTIN ISOFORM IN THE RAT AORTA IN-VITRO

The temporal changes in the expression of fibronectin and other extrac ellular matrix genes were studied in rat aortic rings incubated in vit ro in a serum-free medium. Changes in all forms of fibronectin mRNA in creased progressively during the 24-hour incubation period, although a n increase in the alternatively spliced form of fibronectin designated EIIIA was most pronounced. Both collagen and elastin mRNA levels decr eased markedly during the 24-hour interval, as did alpha-actin mRNA. T he increase in the relative amount of the EIIIA isoform after a 24-hou r incubation was also shown using ribonuclease protection assays. In s itu hybridization showed the distribution of the induced fibronectin m RNA to be within all cell types, including endothelial cells, medial s mooth muscle cells, and adventitial fibroblasts. Localization in the m edia was not uniform and was clearly identified mainly in clusters of cells distributed throughout the media. The early induction of fibrone ctin mRNA was inhibited by genistein, implicating tyrosine kinase acti vation as a causative factor in fibronectin expression. The in vitro c hanges reported may reflect a phenotypic change in vascular cell types that is both similar to and different from the changes reported in vi vo under conditions in which vascular injury and repair occur.